1. Field of the Invention
The present invention relates to a fuel cell. More particularly, it relates to a fuel cell incorporating a sulfonated polysulfone membrane-electrode assembly, and to said sulfonated polysulfone electrolyte membrane-electrode assembly.
Moreover, the present invention relates to an apparatus powered by said fuel cell.
2. Description of the Related Art
Fuel cells are highly efficient electrochemical energy conversion devices that directly convert the chemical energy derived from renewable fuel into electrical energy.
Significant research and development activities have been focused on the development of proton-exchange membrane fuel cells. Proton-exchange membrane fuel cells have a polymer electrolyte membrane disposed between gas-diffusion positive electrode (cathode) and negative electrode (anode), forming the so-called membrane-electrode assembly (hereinafter referred to as “MEA”).
The polymer electrolyte membrane comprises a proton-conducting polymer. Its role is to provide a means for ionic transport and for separation of the anode compartment and the cathode compartment.
Cathode and anode usually contains a metal catalyst supported by an electrically conductive material, for example, platinum (Pt) or alloys thereof, supported on finely divided carbon. Said metal catalyst is combined with a proton-conducting polymer, which can be the same or other than that of the polymer electrolyte membrane.
The gas diffusion electrodes are exposed to the respective reactant gases, the reductant gas and the oxidant gas. An electrochemical reaction occurs at each of the two junctions (three-phase boundaries) where one of the electrodes, polymer electrolyte membrane and reactant gas interface.
In the case of hydrogen fuel cells, the electrochemical reactions occurring during fuel cell operation at both electrodes (anode and cathode) are the following:
anode: H2→2H++2e−;
cathode: ½O2+2H++2e−→H2O;
overall: H2+½O2→H2O.
During fuel cell operations, hydrogen permeates through the anode and interacts with the metal catalyst, producing electrons and protons. The electrons are conveyed via an electrically conductive material through an external circuit to the cathode, while the protons are simultaneously transferred via an ionic route through a polymer electrolyte membrane to the cathode. Oxygen permeates to the catalyst sites of the cathode where it gains electrons and reacts with proton to form water. Consequently, the products of the proton-exchange membrane fuel cells reactions are water, electricity and heat. In the proton-exchange membrane fuel cells, current is conducted simultaneously through ionic and electronic route. Efficiency of said proton-exchange membrane fuel cells is largely dependent on their ability to minimize both ionic and electronic resistivity.
Polymer electrolyte membranes play an important role in proton-exchange membrane fuel cells. In proton-exchange membrane fuel cells, the polymer electrolyte membrane mainly has three functions: a) acting as the electrolyte providing ionic communication between the anode and the cathode; b) separating the two reactant gases (e.g., O2 and H2); and c) performing as electronic insulator. In other words, the polymer electrolyte membrane, while being useful as a good proton transfer membrane, should also have low permeability for the reactant gases to avoid cross-over phenomena that reduce performance of the fuel cell. This is especially important in fuel cell applications wherein the reactant gases are under pressure and the fuel cell operates at elevated temperatures. If electrons pass through the membrane, the fuel cell is fully or partially shorted out and the produced power is reduced or even annulled.
Fuel cell reactants are classified as oxidants and reductants on the basis of their electron acceptor or donor characteristics. Oxidants can include pure oxygen, oxygen-containing gases (e.g., air) and hydrogen peroxide. Reductants can include pure hydrogen, formaldehyde, ethanol, ethyl ether, methanol, ammonia and hydrazine.
Polymer electrolyte membranes are generally based on proton-conducting polymer/s having acidic functional groups attached to the polymer backbone.
At present, perfluorinated (co)polymers, such as Nafion® (Du Pont), based on perfluorosulfonic acid, are the most commonly used as proton-conducting polymer for polymer electrolyte membranes and in electrode construction. They have chemical and physical properties suitable for the demanding fuel cell conditions but this kind of membrane is expensive because of the fluorochemistry involved in the synthesis. Many studies have been carried out to provide cheaper alternatives to these membranes.
Thermoplastic polymers such as polysulfones, polyethersulphones, polyetherketones, polyimides, polybenzimidazole, have been proposed as substitutes of perfluorinated materials, provided that an acidic functional group (e.g., sulfonic acid group, carboxylic acid group and phosphoric acid group) is introduced into the structural unit. These materials met most of the specifications of the fuel cell membranes, namely high protonic conductivity, stability in oxidant and reducing environments and acidic medium, thermal stability, etc. Among the above-mentioned polymers, polysulfone is considered as very interesting due to its low cost and commercial availability.
WO 01/65623 (Commissariat Energie Atomique) discloses a process for preparation of MEA using a thermoplastic material as polymeric material for both membrane and electrodes. All examples are for sulfonated polyimide materials, no examples for sulfonated polysulfone are given although it is claimed that this process can also be used in this case. The process comprises the formation of a solution of the thermostable polymer, casting it on a support, and before complete dry the electrode is placed on the polymer film. No cell performance is shown.
WO 00/15691 (in the name of Victrex Manufacturing Ltd) discloses ion exchange polymers, particularly sulfonated polyarylethersulfones useful as ion conducting membranes of polymer electrolyte membrane fuel cells. These polymers include at least one of the following moieties:
wherein G is, inter alia,
bonded via one or more of its phenyl moieties to adjacent moieties. The Tg of said polymers may be at least 144° C.
WO 01/71839 (in the name of Victrex Manufacturing Ltd) discloses a method of preparing ion exchange polymeric materials, preferably sulfonated, having a formula as reported in WO 00/15691 supra, that are useful as ion conducting membranes of polymer electrolyte membrane fuel cells. Said material has at least some crystallinity or is crystallisable,
WO 01/19896 (in the name of Victrex Manufacturing Ltd) discloses composite membrane for use as an ion-exchange membrane including a conductive polymer having a formula as reported in WO 00/15691 supra, preferably sulfonated. This polymer is preferably cross-linked to reduce its swellability in water.
WO 02/075835 (in the name of Victrex Manufacturing Ltd) discloses a fuel cell and the use of a polymer electrolyte, having a formula as reported in WO 00/15691 supra, which has at least some crystallinity or is crystallisable. The Tg of said polymer may be at least 144° C.
As reported by Jennifer A. Leeson, Michael A. Hickner, and James E. McGrath, in the paper having title “Design, Fabrication, and testing of Membrane Electrode Assemblies Containing Novel ion Containing Copolymers”, Virginia Polytechnic Institute and State University, Materials Research Institute, Department of Chemistry, Blacksburg, Va., USA (Summer Undergraduate Research Program, August 2001), attempts have been made at using sulfonated poly(arylene ether sulphone)s (BPSH-XX copolymers) of general formula
for the ion exchange membrane (proton-conducting material) with Nafion®-based electrodes. The use of different polymers for membrane and electrode causes both poor adhesion and performance problems at the membrane-electrode interface.
This paper shows a performance comparison between a MEA made with the same BPSH for electrolyte membrane and in the catalytic layer, and an all-Nafion® MEA. BPSH MEA performed comparably to Nafion® one.
Feng Wang et al., Journal of Membrane Science, 197 (2002), 231-242 discuss sulfonated poly(arylene ether sulfone) random copolymers as candidates for proton-exchange membranes to be used in fuel cells. These copolymers, therein identified as PBPSH-XX, have the same formula described by Leeson, J. A. et al., supra, with a sulfonation degree ranging between 0% and 60%.
As reported by this paper, greater ion exchange capacities (IECs) are needed with sulfonated poly(arylene ethers) to achieve similar conductivities to perfluorosulfonic acid Nafion® polymers, which is attributed to the strength of the acid group of each system. IEC is based on the quantity of acidic functional groups (e.g. SO3H groups) per dry membrane weight. Nafion® 1135 shows IEC=0.91 meq/g, highly sulfonated PBPSH-40 and PBPSH-60, described therein, have IEC of 1.72 meq/g and 2.42 meq/g, respectively.
Proper hydration of the electrolyte membrane is critical for fuel cell operation. Water uptake increases with sulfonate content due to a strong hydrophilicity of the sulfonate groups. Feng Wang et al. supra reports that the water uptake increases almost linearly from 4.4% for PBPSH-10 to 39% for PBPSH-40 and thereafter increases rapidly to 148% for PBPSH-60.
The water up-take (or water swelling) is to be sufficient for the membrane proton-conductivity, but not so high to cause excessive swelling that leads to a decrease in the membrane strength properties or a membrane deformation, as reported, inter alia, by EP-A-1 138 712 (in the name of JSR Corporation).
This patent application discloses that, although proton conductivity improves with the increasing amount of sulfonic acid groups incorporated, the incorporation of a large amount of sulfonic acid groups results in a sulfonated polymer having considerably impaired mechanical strength properties. Sulfonated polyarylene copolymers with an IEC ranging between 1.5 and 3.5 meq/g are disclosed as satisfactorily performing in hot water, even if comparative compounds having an IEC of about 3 meq/g proved not to retain the membrane shape in hot water. None of the exemplified polymers is a polysulfone.
In addition to IEC and water uptake (WU), glass transition temperature (Tg) of the proton-conducting polymer is of importance in a proton-exchange membrane fuel cell.
MEA are prepared by pressing the electrodes on the polymer electrolyte membrane, normally at a temperature slightly higher than glass transition temperature of the proton-conducting polymer. For example, for all based Nafion® MEAs this temperature is around 130° C. (E. Passalacqua, et al., Electrochimica Acta, 2001, 799).
It is advantageous to work with polymers with low glass transition temperature not only because of their better workability, but also in view of desulfonating process likely to occur, especially at temperatures of about 230-250° C., as from F. Lufrano et al., Solid State ionics, 145 (2001), 47-51.
The lower the glass transition temperature is, the lower the temperature required for pressing the electrodes against the membrane will be. In this way it is avoided a possible degradation of the sulfonic group, being the most sensitive part of the polymer structure.
Also, low Tg values also correspond to higher solubility in solvents, which entails a higher workability.
Sulfonated poly(arylene ether sulphone)s PBPSH-40, PBPSH-50 and PBPSH-60 described by Feng Wang et al., supra, show Tg higher than 270° C. Such Tg values make difficult to use them for assembling a MEA.
Summarizing, it is important for the good performance of proton-exchange membrane fuel cells based on a sulfonated polymer having a degree of sulfonation sufficient to provide high ion exchange capacity (IEC), but not yielding an excessive water uptake (WU). Also, the glass transition temperature (Tg) should be considered for having a proper workability of the polymer in the MEA and a good stability thereof.